The invention relates to salt mixtures for storing thermal energy in the form of heat of phase transformation and to the use thereof.
In technical processes it is a frequent necessity to avoid thermal peaks or deficits. To do so it is common to use heat exchangers. These contain heat transfer media which transport the heat from one location or medium to another. In order to dissipate thermal peaks, for example, the emission of the heat via a heat exchanger to the air is utilized. This heat, however, is then no longer available to compensate the thermal deficits. This problem is solved by the use of heat storage systems.
Examples of suitable storage media are water or stones/concrete, in order to store perceptible (xe2x80x9csensiblexe2x80x9d) heat, or phase change materials (PCMs) such as salts, salt hydrates or mixtures thereof, in order to store heat in the form of heat of fusion (xe2x80x9clatentxe2x80x9d heat).
It is known that the melting of a substance, i.e., its transition from the solid to the liquid phase, involves consumption, i.e., absorption, of heat which for as long as the liquid state persists, is stored in latent form, and that this latent heat is released again on solidification, i.e., on transition from the liquid to the solid phase.
A fundamental requirement for the charging of a heat storage system is a higher temperature than can be obtained in the course of discharge, since heat transport/flux necessitates a temperature difference. The quality of the heat is dependent on the temperature at which it is available: the higher the temperature, the more diverse the uses to which the heat may be put. For this reason, it is desirable for the temperature level in the course of storage to fall as little as possible.
In the case of sensible heat storage (e.g., by heating of water) the input of heat is associated with gradual heating of the storage material (and vice versa during discharge), whereas latent heat is stored and discharged at the melting temperature of the PCM. Latent heat storage therefore has the advantage over sensible heat storage that the temperature loss is limited to the loss during heat transport from and to the storage system.
In order to be able to optimize latent heat stores for the respective utility, it is necessary to have available storage materials which not only have the correct melting temperature but also meet numerous other criteria. These criteria include maximum enthalpy of fusion and heat capacity, low vapor pressure, chemical and physical stability, low corrosiveness, reproducible phase transition, congruent melting behavior, little subcooling, little volume change, high heat conductivity, toxicological acceptability, recyclability, and a low price (J. Schrxc3x6der, Energy Research, 1981, 103-109). For this reason, only a very few technically utilizable storage materials are known. For the majority of temperatures, no suitable materials are yet available.
Storage media for precisely defined temperatures are offered with specific salt mixtures.
U.S. Pat. No. 5,728,316 uses salt mixtures based on magnesium nitrate and lithium nitrate to store and utilize thermal energy, these mixtures, with a melting temperature of 75.6xc2x0 C., being ideally suited to use in latent heat storage systems for use in motor vehicles.
Within the automotive industry, engines are undergoing continual further development. In addition to reductions in fuel consumption and in the emission of pollutants, the operating temperatures of the engines are being optimized further.
It is an object of the present invention, therefore, to provide storage media for different temperatures, having improved properties.